As integrated circuits become smaller and faster, Critical Dimensions (CD's) of devices must decrease. Current state-of-the-art requires critical dimensions of approximately 0.1 micron, and manufacturers are striving to move to lateral dimensions of 65 nm. Consequently, better lithographic resolution is required in order to print smaller features. As per the Rayleigh limit, resolution r is inversely proportional to Numerical Aperture (NA) according to the equation
      r    ∝          λ      NA        ,where λ is the wavelength of the radiation, for a diffraction-limited system. Therefore, to decrease (i.e., improve) resolution for a given wavelength of light, NA must be increased. Larger NA implies a larger collection angle of the lens, i.e., a collection angle of 180 degrees yields the maximum NA of 1. Present steppers have NA close to 1 to provide resolution of minimum size features.
A consequence of increased NA is a decrease in Depth of Focus (DOF), according to the equation
  DOF  ⁢          ∝            λ                        (          NA          )                2              .  Current steppers therefore have decreased DOF due to improved resolution. This presents challenges in lithography, since out-of-focus exposure of features in photoresist smears the resist edges, as illustrated in FIG. 1. Incorrect lateral feature dimensions can result. If defocus is not detected, the microcircuit yields will suffer and the problem may not be detected until many steps later in the manufacturing process.
Steppers are generally equipped with autofocus, which tries to find the best focus for each field of the stepper (usually one die or several dies). However, several factors can cause local or global focus problems:                1) The mounting of the reticle, i.e., the master pattern, may have a tilt. This causes full-field focus problems.        2) The autofocus on the stepper may have a problem, which could cause a full-field defocus. Either of these problems would result in a defocus region of about one inch dimension.        3) Local deformation of the wafer, e.g., caused by contamination on the wafer backside or to structures on the frontside, can cause localized defocus, known as “hot spots”. These may be 50-100 micron diameter.        
One priority for Lithographic After-Development Inspection (ADI) is detection of focus errors in the stepper/scanner, so that corrective action can be taken immediately. Both localized and full-field defocus detection is needed. Traditionally, scanner/stepper defocus has been detected using manual inspection. One often-used detection method used in manual inspection of a wafer which has been patterned and has had the resist developed is to look for color changes across the wafer when observing the wafer under narrow-band diffuse illumination. Color changes result from out-of-focus regions, due to the fact that changing the profile of diffraction grating edges can drastically change its scattering profile and therefore can cause an apparent color change. This is seen from the grating equation:sin(θ)−sin(θ1)=nλ/D  (1), where
θ=angle of observation with respect to normal
θ1=angle of illumination with respect to normal
n=integer order
λ=wavelength
D=grating pitch
Since the repetitive structures on integrated circuits act as diffraction gratings, and since defocused regions have smeared edges as described above, defocused regions are evidenced by color changes. The operator cannot resolve the details of the patterning; he is merely detecting the collective diffractive effects of an area of patterned resist, i.e., “macro-inspection”. From equation (1), it can be seen that variation of either angle or wavelength can affect the appearance of the grating.
The manual observation of color changes to detect defocused regions has severe limitations, due to the tri-stimulus color response of the eye, and its limited gray-scale depth at any wavelength. This is typically compensated for by mounting the wafer on a wobbler, and presenting it to the operator at a variety of angles. The human eye can thereby detect not only a slight color change, but also some “flashing” of the color change as the wafer rotates and wobbles. This method is the most effective for observing localized defocused regions.
Automated macro-inspection systems have also been used to detect defocus, along with other defects, using machine vision, i.e., imaging techniques. Such systems as the Nikon macro-inspection system uses a spin-wobble mount similar to that used in manual inspection, whereby the wafers are tilted and rotated around the azimuth. A high-resolution CCD camera images them through telecentric optics, and image processing is used to detect intensity variations in the observed image. The 2401 and 2430 inspection systems made by KLA-Tencor use narrow-spectral-band and broad-spectral band illumination, use monochrome sensors and detect defocus as an intensity change, and use a line-scan mechanism for imaging.
It is expected that existing automated macro-inspection systems will find it progressively more and more difficult to detect defocus as the CD's shrink, because visible wavelengths are being used, and the diffraction gratings created by the photoresist will have a pitch much smaller than the wavelength of the light used. Shorter wavelength light may damage the photoresist. The development of new methods with increased sensitivity for defocus detection, both for localized and extended defocus defects, will be important as critical dimensions continue to decrease.